This invention relates to a microwave switch which is suitable for controlling microwave energy in a waveguide system. It is particularly difficult to fabricate a microwave switch which is compatible with waveguide structures as this generally requires the provision of high precision moving parts. As an alternative, it has been proposed to utilize a fin line structure, as it is possible to electrically control the conductivity of such a structure without the need to provide mechanically movable parts. Switchable diodes can be used to short circuit opposing edges of a fin line structure, when it is required to render the switch non-conductive. Fin line structures are described in, for example, "Integrated Fin Line Millimeter Components" by P. J. Meier, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 12, December 74. Microwave switches which incorporate fin line structures have not been capable of providing a very high level of electrical performance and in particular in the off state, i.e. when the switch is nominally non-conductive, the impedance which it presents to an applied microwave signal is not sufficiently high for many purposes, as this can have the effect of allowing a relatively low level electrical signal to reach a load. Even though the level of this leakage signal is low, it is very undesirable and quite unacceptable for certain applications. The present invention seeks to provide an improved microwave switch in which this drawback is reduced.